My refrigerator is a MOD.#HSS25GFPDWW freser is frosen ,Do I need the Heater coil or th defrost thermostat?
If the evaporator coils are impacted with ice you will want to do a continuity test on the defrost heater and thermostat. If either does not have continuity then it will need to be replaced. If both check ok then you will want to check the resistance of the defrost thermistor. If the thermistor checks ok then you would need a main control board. If the thermistor does not have proper reading then replace the thermistor. The thermistor resistance reading should be 42.5K ohm at 0 degrees.
You can test this with a multi-meter TJMA-DMT3. Here is a link on how to use a multi-meter. http://forum.appliancepartspros.com/oven-repair-including-ranges-cooktops/4810-how-check-continuity-ohmmeter.html